Life-cycle GHG emissions of electricity from syngas produced by pyrolyzing woody biomass

Abstract

Low-value residues from forest restoration activities in the western United States intended to mitigate effects from wildfire, climate change, and pests and disease need a sustainable market to improve the economic viability of treatment. Converting biomass into bioenergy is a potential solution. Life-cycle assessment (LCA) as a sustainable metric tool can assess the impact of new bioenergy systems. Using the internationally accepted LCA method, this study evaluated the syngas electricity produced via a distributed-scale biomass pyrolysis system called the Tucker Renewable Natural Gas (RNG) system (pyrolysis system developed by Tucker Engineering Associates, Locust, NC). This system converts woody biomass in a high temperature and extremely low oxygen environment to a medium-energy synthesis gas (syngas) that is burned to generate electricity. The pyrolysis process also produces biochar as a byproduct and low-energy (unused) syngas as a waste. Results from the life-cycle impact assessment include an estimate of the global warming (GW) impact from the cradle-to-grave production of syngas for electricity. It showed a notably lower GW impact value (0.142 kg CO₂-eq /kWh) compared to electricity generated from bituminous coal (1.08 kg CO₂-eq /kWh) and conventional natural gas (0.72 kg CO₂-eq /kWh), when the carbon sequestration benefit from the biochar byproduct is included. In addition, the evaluation of the GW impact for Tucker syngas electricity showed the highest GHG emissions came from burning propane to maintain the endothermic reaction in the Tucker RNG unit. Using the previously unused low-energy (waste) syngas to supplement propane use would further reduce GHG emissions (ie fossil CO₂) associated with syngas electricity by 20%.

Keywords

Bioenergy, life-cycle assessment, GHG emission, syngas electricity, woody biomass

Citation

Gu, Hongmei; Bergman, Richard. 2015. Life-cycle GHG emissions of electricity from syngas produced by pyrolyzing woody biomass. Proceedings of the 58th International Convention of Society of Wood Science and Technology June 7-12, 2015 Jackson Lake Lodge, Grand Teton National Park, Wyoming, USA, pp. 376-389.